The automatic switching on and off of a hotplate as a direct function of the placing thereon of a cooking vessel has been a long existing aim, but which has hitherto only been achieved incompletely, with great technical cost and not having the necessary reliability, so that such systems have found relatively little practical application.
The systems proposed for this purpose are based on the most varied principles, the nature and arrangement of the sensor usually being decisive. Thus, mechanical, capacitive, optical, resistive and inductive sensors have been proposed. In inductive sensors both coils with several turns and also those with a single turn have been proposed. These coils are either circular and arranged concentrically to the cooking zone or frame the latter in the case of non-circular cooking zones. These coils are normally located in the vicinity of the marginal insulation (cf. EP 490 289 B1 and EP 442 275 A2).
The aforementioned, single-turn pot detection loop is known from DE 37 11 589 A1. It is a passive short-circuit loop positioned between the heating elements and a glass ceramic plate. It is extraneously supplied by a magnetic field generator located below the heating elements. By periodic short-circuiting and a corresponding damping measurement, the evaluating circuit is energized. The introduction of such a system into practical application has failed due to the considerable cost and in particular the necessarily large overall height for the housing of the magnetic field generator.
The aforementioned multi-turn coils in the outer marginal area (or in an unheated central area) give rise to thermal problems and, as has been recognized by the invention and as will be explained hereinafter, are less suitable for sharp signal generation and detection.